Method for retarding the decomposition of hydrazine rocket fuels in contact with maraging steels

ABSTRACT

Liquid hydrazine fuel is stored in a maraging steel container, wherein the surface exposed to the liquid fuel comprises a bimetallic couple of metallic cadmium and bare maraging steel. The rate of decomposition of the hydrazine fuel based on the exposed area of maraging steel is markedly lower than that for maraging steel alone.

United States Patent Young et al.

[451 May 22, 1973 METHOD FOR RETARDING THE DECOMPOSITION OF HYDRAZINE ROCKET FUELS IN CONTACT WITH MARAGING STEELS Inventors: John P. Young, Gaithersburg, Md.;

Wahling H. Ng, Rockaway, NJ.

Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

Filed: May 28, 1971 Appl. No.: 148,062

US. Cl. ..l49/36, 149/109, 149/3 Int. Cl ..C06c 1/02 Field of Search ..l49/3, 36, 109; 220/64; 204/40, 50; 117/97, 105, 107;

[56] References Cited UNITED STATES PATENTS 3,021,667 2/1962 Griflin et al. 149/36 X 3,086,945 4/1963 3,421,316 1/1969 3,517,508 6/1970 Primary Examiner-Stephen J. Lechert, Jr. Attorney-Harry M. Saragovitz, Edward J. Kelly, Herbert Berl and Victor Erkkila [5 7] ABSTRACT 6 Claims, 1 Drawing Figure Patented May 22, 1973 MANOMETER ZERO METHOD FOR RETARDING THE DECOMPOSITION OF I-IYDRAZINE ROCKET FUELS IN CONTACT WITH MARAGING STEELS The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION The present invention relates to a novel method and apparatus for retarding the decomposition of hydrazine rocket fuels in contact with maraging steels.

The high tensile strength (up to 300,000 psi) and hardness of 18% Ni maraging steels makes possible the fabrication of high pressure tankage systems, which possess less weight and bulk than those made from lower density materials. These properties are desirable for material used to construct liquid propellant tanks for rockets as they increase the ratio of payload to total weight of a flight vehicle and thereby permit improved performance. Maraging steels have been employed as containers for solid propellants. However, their use in conjunction with liquid hydrazine type rocket fuels has encountered problems because of decomposition of the fuel in contact with the maraging steel, which generates gaseous decomposition products and dangerous pressures within the container.

Accordingly, a need has existed for a method for retarding the decomposition of hydrazine fuels in contact with maraging steels.

It has been proposed to coat the maraging steel with a metal which is relatively inert towards liquid hydrazine fuels. However, the choice of metals for this purpose is limited, since it is known that many metals react with or are dissolved by such propellant fuels. Further, in view of the passivity of maraging steels, it is difficult to produce a satisfactory coating thereon with a suitable metal by electrolyte or electroless methods. Also, the irregular interior surfaces and welds usually present in maraging steel tanks increase the difficulty of providing an adequate coating of the metal. Uncoated parts, or very thinly coated areas which are prone to contain pinholes, can be particularly deleterious, since with some metals maraging steel forms a bimetallic couple, which actually accelerates the decomposition of the hydrazine fuel.

It is therefore an object of this invention to provide a method for retarding the decomposition of liquid hydrazine type fuels in contact with maraging steels.

Another object is to provide a bimetallic couple with a maraging steel, which reduces the rate of decomposition of liquid hydrazine fuels in contact with maraging steel.

A further object is to provide a maraging steel container for liquid hydrazine type fuels, wherein the surface exposed to said fuel comprises another metal in contact with said maraging steel, thereby providing a bimetallic couple which inhibits the decomposition of the hydrazine fuel by the maraging steel.

Other objects are either obvious or will appear from the description of the invention below.

DESCRIPTION OF THE INVENTION These objects are achieved in accordance with our novel method for retarding the rate of decomposition of liquid hydrazine fuels in contact with maraging steels, which comprise incorporating metallic cadmium with the maraging steel surface exposed to the liquid hydrazine fuel. We have discovered that liquid hydrazine fuels in contact with a surface composed of a maraging steel-cadmium couple decompose at a substantially slower rate, based on the area of maraging steel exposed, than when in contact with a surface consisting of the maraging steel alone. This inhibiting action is quite unexpected, since we have found that other metals e. g. nickel and silver, which catalyze the decomposition of such fuels at a slower rate than maraging steel, when coupled with maraging steel greatly accelerate the decomposition of said fuels by the maraging steel.

The protective action of the maraging steel-cadmium couple is not clearly understood. It is theorized that such action is the result of an electrolytic bimetallic couple, wherein the catalytic effect of the maraging steel on the decomposition of hydrazine fuels is cancelled by the anodic effect of cadmium, in analogy to an aqueous system, wherein cadmium is anodic to steel and exerts a protective electrolytic effect with respect to corrosion.

Another surprising aspect of the invention is the large distance, e.g. several centimeters, over which cadmium is capable of inhibiting or reducing the catalytic decomposition of hydrazine fuels at a contiguous maraging steel surface. This property is particularly valuable for protecting the surfaces of maraging steel tanks, where the entire surface for some reason cannot be or is not completely coated (e.g. due to pinholes in the plated metal coating or due to the inability to completely coat the interior of the tank because of the complex and irregular configuration of the tank surface, welded joints, etc.), or if further work on the tank subsequent to plating, e.g. welding, should damage some areas of the coating. A further advantage of the invention is the discovery that cadmium does not undergo any significant corrosion by hydrazine fuels.

The cadmium can be incorporated with the maraging steel in any suitable manner, such as for example by electroplating a thin coating of said metal on the maraging steel, attaching metallic cadmium sheets, strips, etc. to the maraging steel by welding, riveting, etc.

Liquid hydrazine fuels suitable for use in our invention include hydrazine, monomethyl hydrazine, 1,1- dimethyl hydrazine (unsymmetrical dimethyl hydrazinc) and mixtures thereof.

Maraging steels as a class are high nickel steels of low carbon content, whose strength is achieved primarily through aging a martensitic matrix. Especially high strength maraging steels contain, besides iron, substantial amounts, e.g. 10 to 20 percent by weight, of nickel together with lesser amounts of cobalt and molybdenum, and small amounts of titanium and aluminum, the cobalt, molybdenum, titanium and aluminum serving as hardening agents which increase the strength of the steel. Maraging steels containing significant amounts of cobalt or molybdenum, in view of their high strength to weight ratio, are particularly desirable for fabrication of rocket motor cases for liquid hydrazine fuels. Hence, our invention is especially valuable for use with such maraging steels, since we have found that metallic cobalt and molybdenum greatly accelerate the decomposition of liquid hydrazine fuels. The following illustrate maraging steels suitable for use in our invention:

Maraging steel Nominal Chemical Nominal Yield type Composition Strength Ni Co Mo Ti Al l8 Ni 300 18 9 .7 l 300,000 psi l8 Ni 250 18 7 S .5 .1 250.000 psi 18 Ni 200 18 8.5 3.5 .2 .1 200,000 psi 18 Ni 180 I8 8 2 .l5 1 180,000psi l2-2Ni I 8 4 .2 1 200,000 psi Maraging steel specimens were partially and completely coated with cadmium and other metals as described in the following examples and the resulting specimens together with control specimens of maraging steel and other metals were tested for their effect on the decomposition of hydrazine rocket fuel, as described below under Test Procedure". The accompanying drawing illustrates a glass apparatus used in the tests for measuring gas generated from decomposition of the fuel. The test results are set forth in the Table below.

Test Procedure In service, rocket fuels may be stored in deserts at temperatures that may reach 160F or in artic areas where temperatures may drop as low as 65F. Since elevated temperatures would be expected to cause maximum rates of decomposition of the fuel, the tests were conducted at 160F. The decomposition of the fuel results in the continuous generation of gas, which over a long period of time, could reach a pressure sufficient to rupture a tank. Generally, for safety reasons, the internal pressure buildup in a rocket tankage system cannot be permitted to exceed 100 psi per year or a total of 500 psi over a period of five years. Since the desired storage life of the rockets is about 6 years, the tests were continued for a period of at least one year to predict long-term performance.

The glass unit illustrated in the drawing was employed for measuring gas evolved from decomposition of fuel. It consists of a bulb 1 connected via vertical tube 2, cross-arm tube 3 and downwardly extending vertical tube 4 to a manometer loop consisting of bulb 5 and tube 6 containing mercury 7. A glass frit 8 prevents accidental transfer of mercury to bulb 1 containing the test specimen 9 immersed in the fuel 10. The apparatus is strengthened by braces 11 and 12.

The dotted extensions 13 and 14 of specimen bulb 1 and fuel inlet tube 2 resp. show the form of apparatus prior to loading. The unit in this form was tagged with an identification number and the following calibration measurements made: (a) Volume per unit length of the right-hand leg of the manometer. (b) Total manometer head as a function of height of mercury above the zero point in the right-hand leg. (c) Volume per unit length of the specimen chamber. (d) Volume of the crosshatched portion of the gas chamber.

These data, combined with manometer readings, are required for calculating the quantity of gas formed.

The unit was then cleaned by the following steps: (a) Submerge and soak in a nitric acid solution (1:1 dilution of conc. HNO for 2 hours. (b) Rinse with distilled water. (c) Soak two hours in aqueous NH:, l part conc NH plus 4 parts H O). (d) Rinse with distilled water. (e) Oven dry.

The unit was then clamped in an inverted position. The specimen to be tested (previously cleaned by usual pre-plating procedures, followed by a soak in the above NH solution, rinsed and air-dried) was inserted into the specimen chamber through its open end. The specimen chamber was then sealed by fusing and drawing the glass. Adequate precautions were taken during this operation to avoid heating and oxidizing the specimen (Step-wise drawing with intermittent cooling, air blast cooling of the outside of the opposite end of the specimen chamber, and simultaneous flushing of the specimen chamber with nitrogen). The fuel inlet tube was then sealed at a point a few centimeters above the position of the seal shown in the drawing. The unit was then pumped down to a low pressure through the open end of the manometer tube and all seals were checked with a spark-type, high voltage leak tester. Leaks, if found, were repaired. For further leak-testing, mercury was then added to the manometer and the unit was pressurized with N using a 2 mm TFE tube pushed through the mercury column, to the limit of the manometer tube, thus exposing all fusion seals and joints to the gas. After about two weeks, if no evidence of a leak was observed the units were opened by breaking the fuel inlet tube just below its sealed tip and enough fuel was added to cover the specimen. A plastic syringe with its needle attached to a 2 mm-dia TFE tube was used to introduce the fuel into the lower end of the specimen bulb without wetting the wall of the inlet tube. A temporary closure of the inlet tube was then made with a TFE plug. The fuel was left in the unit about one week to leach possible residual impurities from the specimen and the interior of the unit. This fuel was then removed by use of the syringe and replaced with fresh fuel, again avoiding wetting of the upper end of the inlet tube. If wetted at this point, it was sometimes difficult subsequently to obtain a leak-free fusion seal. The fuel inlet tube was then resealed by fusion at the position shown in the drawing. The seal was inspected under magnification. lf perfect, the unit was flushed (alternately pressurized and depressurized) with N by inserting a long, 2 mmdia TFE tube through the open end of the manometer, through the mercury, and into the gas chamber above the mercury in the manometer bulb. The unit was then placed in the constant temperature bath to begin the test.

Measurement Procedures Rate of gas formation A scale graduated in centimeters was attached to the manometer tube of the glass units to permit measurement of the height of the mercury at given times. From these values and the calibration data for the unit, pressure and gas volume in the unit were determined.

For the glass units, the equation (V2172 ipolpfl gives the amount of gas evolved, measured at atmospheric pressure (p where V and V are initial and later gas volume in the unit, respectively, and p and p are the corresponding pressures. The amount of gas evolved, as calculated from equation l is due in part to catalytic decomposition of fuel at the surface of the metallic specimen, and in part to decomposition of fuel at other locations, e.g., in the liquid itself, in the vapor phase, and at glass-fuel interfaces. Decomposition at these locations is referred to as background decomposition". It was determined in identical units except that no metal specimen was present. AV due to background decomposition, which was found to be very small in the glass units, was subtracted from AV of equation (1) to obtain the volume of gas evolved due to the effect of the metallic specimen itself.

Since the rate of gas evolution due to contact of a given metal with the fuel is proportional to the area of the metal, catalytic activity may be expressed as a rate coefficient as follows:

=cm. /cm. days The following examples illustrate the invention:

EXAMPLE 1 (A). A maraging steel specimen approximately 1 cm. wide, 5 cm. long, 2 mm. thick with a surface area of 15 cm was plated with cadmium as described below. The specimen was cut from a sheet of heat-treated maraging steel of the following chemical composition in addition to iron: Nil8, Co 9, Mo 5 and Ti 0.6%.

Because of its passivity due to substantial content of cobalt and molybdenum, maraging steel requires special preplating processing to activate its surface and remove oxide films which interfere with proper adhesion of a metal deposit. Consequently, the heavy heattreatment scale was first removed by mechanical abrasion and pickling in an aqueous mixture of concentrated sulfuric and nitric acids. After descaling, the maraging steel specimen was treated in the manner described below to apply a nickel undercoat or strike with an acid nickel chloride solution, which is more reactive with the maraging steel surface than standard plating solutions. The treatment represents a modification of the procedure described in US. Pat. No. 3,338,803, G.A.D. Bari.

l. Pumice scrub.

2. Water rinse.

3. Electropolish, until surface is smooth and clean, in a solution of 20 per cent (by wt) of H 80 60 per cent H PO 5 per cent CrO 15 per cent H O. Temperature 40C (104F); anode current density, 7 A/dm; duration, 2 min.

4. Water rinse. 5. Remove smut, if necessary, by immersing for one minute in chromic sulfuric acid: CrO 50 g/l; H SO g/l; room temperature.

6. Water rinse.

7. Strike in an acid nickel chloride solution: NiCl -6- H 0, 240 g/l; HCl (conc), 125 ml/l; room temperature; cathode current density, 2 A/dm plating period, 1 min. Immerse for l min before applying current.

8. Rinse and transfer quickly to the desired plating bath.

The thickness of the final deposit was determined by weighing the specimen before step l above and after plating, after applying corrections for weight changes due to steps 3 and 7. The weight after plating also served as a basis for determining weight loss due to corrosion resulting from subsequent exposure of the specimen to the fuel.

The cadmium plate was deposited on the nickel undercoat produced in steps 7 and 8 using the plating bath and conditions described below:

CdO 22.5 g/l NaCN 99.6 3/1 NaOH 14.0 g/l Na,CO 4.0 g/l Room temperature 20 amp/ft. Thickness of Cd coating 0.002 in.

After plating the specimen was rinsed with water and dried.

EXAMPLE 2 An identical maraging steel specimen was platedonly over half its length with cadmium on the nickel strike in the foregoing manner leaving the other half of the specimen surface bare maraging steel.

Example 3 A cadmium plated specimen obtained as described in Example 1 was provided with simulated pores by drilling just deep enough to expose the maraging steel. The porous specimen was prepared to simulate the probable porosity encountered in production tanks due to imperfections in the plate, base metal, welding defects, etc.

EXAMPLE 4 Cadmium sheet caps 4 mm. long were pinched onto each end of a bare maraging steel specimen identical with that employed in Example 1.

EXAMPLE 5 Two maraging steel specimens of the foregoing type were plated with nickel by an electroless method; one specimen was completely coated by the nickel plate while the other was coated over only one-half its length leaving the other half bare maraging steel. The surface preparation of the maraging steel specimens was modified in that an activating acid treatment, consisting of a 20 minute soak with 10% aqueous hydrochloric acid, was applied following the electropolishing step. Thereafter the maraging steel specimen was coated with elec- In similar manner two maraging steel specimens were given the same surface preparation and nickel strike as described above and then completely coated with silver from a standard electroplating bath. One of the plated specimens was provided with simulated pores by drilling just deep enough to expose the maraging steel.

Data on Decomposition of Mixed Hydrazine- Monomethyl-l-lydrazine Fuel at F 3:2; Gas Rate Description lest evolved (a) coefficient Ex. (Deposits 2 mils thick) days cm cm"/day/cm'-' l Cadmium on MS area of specimen 14.7 cm 6l4 l.l 0.00012 2 Cadmium on Maraging Stee1(MS) with A (8 cm) of MS surface exposed 601 46.3 0.0096(b) 3 Cadmium on MS with 200 pores of 0.026" dia. MS area exposed 1.04 cm 493 l 1. 0.021805) 4 Cadmium sheet caps 4 mmv long pinched onto each end of a 1 X 5 cm MS sheet 210 51.6 0.0284(17] 5 Electroless nickel on MS with Be of MS surface exposed 14 cm 252 177.3 0.183501) Electroless nickel on MS area (a) Cumulative total, corrected for background rate and to 1 atm. pressure (b) Rate coefficient is calculated on basis of exposed MS only.

(c) Solid metal specimens.

DISCUSSION OF TEST RESULTS For specimens containing exposed area of maraging steel, the decomposition rates of the hydrazine fuel were calculated on the basis of the area of maraging steel exposed. lt was found that the rate of fuel decomposition was nearly proportional to the area of maraging steel exposed.

The results reveal the surprising fact that fuel exposed to the cadmium-maraging steel couple decomposed at a considerably slower rate, based on the area of maraging steel exposed, than that exposed to maraging steel alone, whereas the opposite effect was obtained with both the nickel-maraging steel and silvermaraging steel couple. in fact. the latter couples greatly increased the rate of decomposition of the fuel, by about 100 percent in the case of the nickel-maraging steel couple and about 1,000 percent for the silvermaraging steel couple, as compared with the rate obtained with maraging steel alone.

It is evident from the foregoing that cadmium is a highly preferred material for coating the surface of maraging steel in view of its surprising ability to reduce the catalytic activity of contiguous uncoated areas of maraging steel. While cadmium undergoes some corrosion by such fuels, the rate is so low that it is not consid ered a deterrent to its use. For example, the concentration of dissolved salts in the mixed hydrazinemonomethyl hydrazine fuel from the test unit wherein the specimen was exposed for about one year at F, as determined by evaporation and weighing the residue, was about 0.19 g/liter for cadmium, 0.12 for maraging steel, 0.17 for nickel, and 0.03 for silver, while at 72F for the same period the values were 0.2 g/liter for cadmium, and 0.7 for maraging steel.

We wish it to be understood that we do not desire to be limited to the exact method and detail of construction described for obvious modification will occur to persons skilled in the art.

What is claimed is:

l. A method for retarding the decomposition of a liquid hydrazine fuel in contact with a maraging steel surface, which comprises contacting metallic cadmium (a) with said maraging steel surface or (b) with a metallic nickel coat on said maraging steel surface, such that both the cadmium and the maraging steel are in contact with said fuel, whereby the decomposition of said fuel by said maraging steel is retarded.

2. A method according to claim 1, wherein the maraging steel contains a metal of the group consisting of cobalt and molybdenum.

3. A method according to claim 2 wherein the maraging steel contains 10-20% nickel, 7-9% cobalt and 25% molybdenum.

4. A method according to claim 2 wherein the fuel is selected from the group consisting of hydrazine, monomethyl hydrazine, 1,l-dimethyl hydrazine and mixtures thereof.

5. A method according to claim 4 wherein the fuel is a mixture of hydrazine and monomethyl hydrazine.

6. A method according to claim 3 wherein the cadmium is electroplated on a metallic nickel base coat on the maragin g steel. 

2. A method according to claim 1, wherein the maraging steel contains a metal of the group consisting of cobalt and molybdenum.
 3. A method according to claim 2 wherein the maraging steel contains 10-20% nickel, 7-9% cobalt and 2-5% molybdenum.
 4. A method according to claim 2 wherein the fuel is selected from the group consisting of hydrazine, monomethyl hydrazine, 1, 1-dimethyl hydrazine and mixtures thereof.
 5. A method according to claim 4 wherein the fuel is a mixture of hydrazine and monomethyl hydrazine.
 6. A method according to claim 3 wherein the cadmium is electroplated on a metallic nickel base coat on the maraging steel. 